In the MS radio locating area, usually two locating methods are used: the Time Of Arrival (TOA) method and the Time Difference Of Arrival (TDOA) method. For locating a MS, the TOA method needs three or more than three TOA values, and the TDOA method needs two or more than two TDOA values. In general, a TOA value or TDOA value is obtained by measuring arrival time or arrival time difference of downlink signals from different base stations to the MS. No matter the measurement is an arrival time or an arrival time difference of downlink signals, how a MS captures and detects effectively the first path of downlink signals from different base stations should be concerned.
In W-CDMA, a MS locating with TDOA needs to measure the arrival time difference of downlink signals from different base stations, i.e. making OTDOA (Observed TDOA) measurement in 3GPP. The basic procedure of OTDOA is as follows: the Mobile Location Center initiates to the MS a OTDOA measurement request for designated reference base station; after the MS has received the measurement request, the MS detects the first path of the downlink signal of the reference base station and at the same detects the downlink signal of other neighbor base stations. The MS sets appropriate search window according to the search window parameters in the measurement request to detect the downlink signals of the neighbor base stations. The general search window setting method for the neighbor base stations is as follows: take the location that corresponds the first path of the detected reference base station downlink signal as the center of the search window, take double distances, which can be represented as a propagation time, between a neighbor base station and the reference base station as the search window width; for every neighbor base station, set the search window in the same way, capture and detect the first path of downlink signals, and then subtract or make correlation operation with the downlink signal first path of the reference base station to obtain appropriate time-delay estimated TDOA value.
Advantages of the method mentioned above are that it is simple to set search window and can capture the neighbor signals completely. The disadvantages are as follows: since its search window is wider, so determination of the first path in the search window is more difficult; and the determination correctness is lower which is that: at conditioning of the same noise threshold, with a wider search window, the false alarm probability is higher, i.e. the probability of seeing a noise as a first path is higher. Besides, in a locating measurement it is more concerned about the accuracy of first path location but not about whether the captured multipath signals are completed; this is totally different with the aim of multipath searching with a Rake receiver. When the first path signal cannot be detected because of small size fading and the time-delay between the second path signal and the first path signal is larger, the conventional first path signal measurement method will take the second path as the first path, in this case there is a larger measurement error that is meaningless for location estimation. Therefore, the conventional first path detection method has the disadvantages of a larger false alarm probability and a large error of the first path detection that will decrease MS locating accuracy.